Crosslinkable bioplasticizers

ABSTRACT

A crosslinkable bioplasticizer is disclosed, a mixture of an epoxy-functional bio-derived plasticizer and an aromatic dianhydride. When heated, the mixture crosslinks, which can be accelerated by the addition of a Lewis Acid metal catalyst. The crosslinked bioplasticizer can be melt compounded into a thermoplastic resin or can be formed in situ in the thermoplastic resin. The crosslinking of the bioplasticizer can reduce blooming of the bioplasticizer to the surface of a plastic article made by extrusion, molding, calendering, or thermoforming techniques.

CLAIM OF PRIORITY

This application claims priority from U.S. Provisional PatentApplication Ser. No. 61/512,330 bearing Attorney Docket Number 12011017and filed on Jul. 27, 2011, which is incorporated by reference.

FIELD OF THE INVENTION

This invention relates to sustainable plasticizers from renewableresources which can be crosslinked.

BACKGROUND OF THE INVENTION

All industrial, construction, and consumer products strive to identifyraw materials from renewable resources grown or otherwise harvested fromthe plant or animal kingdom. The expense and increasing scarcity ofpetrochemically originating raw materials only accentuate thedifficulties of recycling after useful life of products made from suchraw materials.

The polymer industry, which had started in the early 20^(th) Centurywith renewable resources such as natural latex for rubber goods, is nowreturning to such renewable raw materials whenever possible.

One body of research aims at bio-derived plasticizers. An example isU.S. Pat. No. 6,797,753 (Benecke et al.), incorporated by referenceherein, which discloses plasticizers derived from vegetable oils.Another is U.S. Pat. No. 7,196,124 (Parker et al.) which disclosesmaking elastomeric materials from castor oil and epoxidized soybean oil.

SUMMARY OF THE INVENTION

While plasticizers are often suitable for a wide variety ofplasticization purposes with a wide variety of thermoplastic resins,(particularly polylactic acid (PLA) and polyvinyl chloride (PVC) fromthe bio-derived and petroleum-derived categories, respectively), thereare situations in which the migration of plasticizer within thethermoplastic resin can cause undesired “blooming” of the plasticizer tothe surface of a plastic article.

Therefore, what the art needs is crosslinkable bioplasticizers. Beneckeet al. mention the use of crosslinking aids with their plasticizers fromvegetable oils but do not specify which aids are suitable.

The present invention solves that problem by mixing an aromaticdianhydride with an epoxy-functional bioplasticizer, in order that suchcombination is capable of crosslinking upon heating, preferably in thepresence of a Lewis Acid metal catalyst.

Therefore, one aspect of the present invention is a crosslinkablebioplasticizer mixture, comprising (a) an epoxy-functional bio-derivedplasticizer and (b) an effective amount of an aromatic dianhydride,wherein when heated, the mixture crosslinks to form a crosslinkedbioplasticizer. Preferably, the crosslinking is accelerated by thepresence of a Lewis Acid metal catalyst and heat.

Another aspect of the present invention is a thermoplastic compoundcomprising (a) a thermoplastic resin and (b) the crosslinkedbioplasticizer identified above.

Another aspect of the present invention is a method of making thecrosslinked bioplasticizer identified above, comprising the steps of (a)mixing the aromatic dianhydride and the epoxy-functional bioplasticizerin a heated environment and optionally, (b) introducing an effectiveamount of Lewis Acid metal catalyst, wherein the aromatic dianhydrideand the epoxy-functional bioplasticizer react to form the crosslinkedbioplasticizer.

Features and advantages of the invention will be explained in respect ofthe various embodiments with reference to the following drawings.

EMBODIMENTS OF THE INVENTION

Epoxy-Functional Bioplasticizer

Any epoxy-functional bioplasticizer is a candidate for use in thisinvention. The extent of epoxy functionality helps determine whichbioplasticizer should be used or how much crosslinking is desired in thecrosslinked bioplasticizer.

U.S. Pat. No. 6,797,753 (Benecke et al.), incorporated by referenceherein, recites both generically and specifically preferredepoxy-functional bioplasticizers. Generically, they can be identified asplasticizers comprising a fatty acid product derived from a vegetableoil having at least 80% by weight of unsaturated fatty acids, whereinsaid unsaturated fatty acids are substantially fully esterified with amonool or a polyol, and said esterified unsaturated fatty acids havebeen substantially fully epoxidized.

Alternatively stated, preferred epoxy-functional bioplasticizers can beidentified as monoesters or multiesters of epoxidized vegetable oils.

Typically the oil has an iodine value (I.V. value), which is ameasurement of the amount of double bonds in the fatty acids of the oil,that is about 100 and higher.

As Benecke et al. explain, epoxy-functional fatty acid esters can beobtained from a number of vegetable oils, such as soybean oil (I.V.value about 120-143), canola oil (I.V. value about 100-115), corn oil(I.V. value about 118-128), linseed oil (IV. value about 170-200),rapeseed oil (I.V. value about 100-115), safflower oil (I.V. value about140-150), sunflower oil (I.V. value about 125-140), tall oil (I.V. valueabout 140-190), and tung oil (I.V. value about 180), and mixtures andderivatives thereof.

All of these fatty acid esters mentioned in Benecke et al. have anadequate number of unsaturated fatty acids (e.g., oleic, linolenic,linoleic) which are suitable for epoxidation, i.e., establishment of theepoxy functionality needed for bioplasticizers useful in this invention.

Preferred oils include any vegetable or plant fatty acid glyceride thatis significantly unsaturated. Significantly unsaturated means that thevegetable oil typically has more than about 80 weight percentunsaturated fatty acids. Most preferably the unsaturation should beabout 84 wt. % or higher.

Naturally occurring products are seldom pure and isolated. The oils usedto make epoxy-functional bioplasticizers have a random mix ofunsaturated fatty acids present in the vegetable oil. Also, thesaturated fatty acids are likewise selected from the random mix ofsaturated fatty acids present in the vegetable oil. The identifyingportions of saturated fatty acids present are termed saturated acylgroups that are derived from saturated fatty acids and are typified bypalmitoyl, stearoyl, arachidoyl, behenoyl, myristoyl, and margaroyl.

Of the various epoxy-functional fatty acid esters, soyates are preferredwith non-limiting examples of such epoxidized soyates including (i)epoxidized pentaerythritol tetrasoyate; (ii) epoxidized propylene glycoldisoyate; (iii) epoxidized ethylene glycol disoyate; (iv) epoxidizedmethyl soyate; (v) epoxidized sucrose octasoyate; and (vi) theepoxidized product of soybean oil interesterified with linseed oil.

Of those listed, epoxidized methyl soyate (EMS) is the most commerciallyavailable as Nexo E1 brand epoxidized methyl soyate from NexoleumBioderivados, Ltda. Cotia, Brazil; as Vikoflex 7010 plasticizer fromArkema, Inc.; and as reFlex 100 bioplasticizer from PolyOne Corporation.

Other epoxidized alkylene monosoyates or multisoyates are lesscommercially available presently but will eventually become significantbioplasticizers in commerce and are predicted to be useful in thisinvention.

Aromatic Dianhydrides

Epoxy functionality on the bioplasticizer might be assumed to havesufficient reactivity to permit self-crosslinking of the bioplasticizer.But that was not found to be currently possible.

Therefore, another chemical is needed to participate in creation ofcrosslinks between and among the epoxidized fatty acid esters.Originally, an aliphatic acid, an aliphatic acid anhydride, and anaromatic acid dianhydride were considered for use in this invention. Ofthem, only the aromatic acid dianhydride was successful, preferably inthe presence of a Lewis Acid metal catalyst.

Dianhydrides are molecules containing two acid anhydride functions. Thesuccessful aromatic acid dianhydride, also known as an aromaticdianhydride, is commonly known as pyromellitic dianhydride (PMDA) andtechnically known as 1,2,4,5-benzenetetracarboxylic dianhydride havingCAS No. 89-32-7.

PMDA is a white or beige powder having a melting point of about 284° C.,a molecular weight of 218.12, and an acid value of 1015 mg KOH/g. It iscommercially available from a number of sources including Sigma-AldrichChemicals.

The formula of PMDA is shown below.

PMDA readily mixes with agitation into the epoxy-functionalbioplasticizer and can even dissolve upon application of heat to themixture.

The effective amount of PMDA can range from about 1 to about 50 partsper one hundred parts (PHR) of the bioplasticizer, desirably between 1and 15 parts, and preferably between 5 and 15 parts. For a less denselycrosslinked bioplasticizer, which also forms a more rubbery solid, lessPMDA (between about 1 and about 10 parts) can be used. Between 1 partand 5 parts, at 2 or 3 or 4 parts, the amount of crosslinking and theextent of rubberiness can exhibit a smooth trend or an abrupt inflectionpoint. But these lesser amounts of PMDA present in the mixture also slowthe reaction of crosslinking. Thus, for a person having ordinary skillin the art, without undue experimentation, one can choose amidst therange of concentration of PMDA to achieve a desired result, recognizinga balance between processing efficiency and performance properties.

The invention also benefits from the relative unreactivity of theepoxy-functional bioplasticizer and aromatic dianhydride until thatmixture is heated, optionally in the presence of the catalyst. Thus, itis possible to have a crosslinkable bioplasticizer which does notcrosslink until a later, controlled time. Then, the crosslinkingreaction between the epoxy-functional bioplasticizer and the aromaticdianhydride can occur in a manner analogous to the rubber industry wherevulcanizable rubber is “green” until a subsequent curing event calledvulcanization.

Optional Catalyst

A Lewis Acid metal catalyst in an effective amount, less than 5 PHR, canbe used to accelerate the crosslinking reaction between theepoxy-functional bioplasticizer and the aromatic dianhydride. Any LewisAcid metal catalyst is a candidate for use in the invention. Butyl tintris(2-ethyl hexanoate) was used in the examples below, available asFasCat 4102 from Arkema, Inc.

Optional Additional Plasticizers

While not preferred in the present invention, it is possible that anadditional plasticizer could be used in addition to the bioplasticizersidentified above. For example, organic esters of various acids such asphthalic, phosphoric, adipic, sebacic, citric, and the like can beadded, optionally. Specific examples of possible additional plasticizersinclude dioctyl phthalate, dioctyl adipate, dibutyl sebacate, anddinonyl phthalate and glyceryl stearates.

Compounds of the Bioplasticizer and Polymer Resins

Bioplasticizers of this invention are useful as functional additives inpolymer resins, preferably thermoplastic polymer resins. Plasticizationof polymer resins is a very well known polymer science activity, torender the plastic articles made from such compounds more flexible,fluid, pliable, soft, etc.

Any polymer resin is a candidate for plasticization by the crosslinkablebioplasticizers of this invention. Preferably, the polymer is athermoplastic. Non-limiting examples of polymers suitable forplasticization include polyvinyl chloride (PVC), polylactic acid (PLA),poly(meth)acrylates (such as polymethylmethacrylate (PMMA)), etc.

PVC Resins

The polymer processing art is quite familiar with vinyl plastisols. ThePVC resins used are typically dispersion-grade poly(vinyl chloride)(PVC) resins (homopolymers and copolymers). Exemplary dispersion-gradePVC resins are disclosed in U.S. Pat. Nos. 4,581,413; 4,693,800;4,939,212; and 5,290,890, among many others such as those referenced inthe above four patents. Any PVC resin which has been or is currentlybeing used to make industrial goods, such as sheet flooring products, isa candidate for use in the present invention. Without undueexperimentation, one skilled in the art can determine gel point, gelrate, and other gelation properties of a PVC resin in performance with acrosslinked bioplasticizer identified above.

In a similar manner, the polymer processing art is also quite familiarwith solid vinyl resins, such as used to make tile flooring.

Vinyl resins useful for tile flooring comprise essentially a homopolymerwith minimal amounts of less than about 5% by weight copolymerized othervinyl comonomer, but preferably little or no copolymerized other vinylmonomer. Commercial PVC resin ordinarily comprises about 56% by weightchlorine and has a Tg of about 81° C.

Preferred PVC resins are essentially homopolymers of polymerized vinylchloride. Useful vinyl co-monomers if desired include vinyl acetate,vinyl alcohol, vinyl acetals, vinyl ethers, and vinylidene chloride.Other useful co-monomers comprise mono-ethylenically unsaturatedmonomers and include acrylics such as lower alkyl acrylates ormethacrylates, acrylic and methacrylic acids, lower alkyl olefins, vinylaromatics such as styrene and styrene derivatives, and vinyl esters.Useful commercial co-monomers include acrylonitrile, 2-hexyl acrylate,and vinylidene chloride. Although co-monomers are not preferred, usefulPVC copolymers can contain from about 0.1% to about 5% by weightcopolymerized co-monomer, if desired.

Preferred PVC resins for tile flooring are suspension polymerized vinylchloride monomer, although mass (bulk) and dispersion polymerizedpolymers can be useful, but are less preferred. PVC resins can have aninherent viscosity from about 0.45 to about 1.5, preferably from about0.5 to about 1.2, as measured by ASTM D 1243 using 0.2 grams of resin ina 100 ml of cyclohexanone at 30° C.

Vinyl plastisols for sheet flooring are typically liquid at roomtemperature and can be poured, pumped, sprayed or cast, depending on theformulation. These compounds can range in hardness from fishing lureplastisol with an 8 Durometer Shore A or lower, to rotocasting plastisol(mostly PVC) with a 65 Durometer Shore D and above. Advantages of vinylplastisol in coating and sheet forming applications include ease of useand economy.

Vinyl compounds for tile flooring are nearly rigid chips or pellets andare calendered into final shape before cutting into tile sizes.

Polylactic Acid

Another thermoplastic resin benefiting from the use plasticizers is PLA.PLA is a well-known biopolymer, having the following monomeric repeatinggroup:

The PLA can be either poly-D-lactide, poly-L-lactide, or a combinationof both. PLA is commercially available from NatureWorks, LLC located inall manufacturing regions of the world. Any grade of PLA is a candidatefor use in the present invention. The number average molecular weight ofPLA can be any which is currently available in a commercial grade or onewhich is brought to market in the future. To the extent that a currentend use of a plastic article could benefit from being made from PLA,then that suitable PLA could be the starting point for constructing acompound using PLA and the crosslinkable bioplasticizer.

Other Optional Additives

A variety of ingredients commonly used in the plastics compoundingindustries can also be included in the compound of plastic resin andcrosslinkable bioplasticizer of the present invention. Non-limitingexamples of such optional additives include blowing agents, slip agents,antiblocking agents, antioxidants, ultraviolet light stabilizers,quenchers, plasticizers, mold release agents, lubricants, antistaticagents, fire retardants, frothing agents, and fillers such as glassfibers, talc, chalk, or clay.

Any conventional colorant useful in coatings and paints or plasticscompounding is also acceptable for use in the present invention.Conventional colorants can be employed, including inorganic pigmentssuch as titanium dioxide, iron oxide, chromium oxide, lead chromate,carbon black, silica, talc, china clay, metallic oxides, silicates,chromates, etc., and organic pigments, such as phthalocyanine blue,phthalocyanine green, carbazole violet, anthrapyrimidine yellow,flavanthrone yellow, isoindoline yellow, indanthrone blue, quinacridoneviolet, perylene reds, diazo red and others.

Table 1 shows the acceptable, desirable, and preferable ranges ofamounts, in weight percents, of thermoplastic resin, crosslinkablebioplasticizer, and optional additives. All amounts are expressed inweight percents. The compounds can comprise, consist essentially of, orconsist of these ingredients.

TABLE 1 Formulations Ingredient Acceptable Desirable PreferableThermoplastic Resin 50-75 55-73 60-70 Crosslinkable 15-40 17-35 20-30Plasticizer Optional Additives  0-25  5-20 10-15

Processing of the Compound

Mixing of Thermoplastic Resin and Plasticizer for Plastisol

Conventional mixing equipment is used to thoroughly mix the plastisol,either in batch or continuous operations.

Mixing in a batch process typically occurs in a low shear mixer with aprop-type blade operating at a temperature below 37° C. The mixingspeeds range from 60 to 1000 rpm. The output from the mixer is a liquiddispersion ready for later coating on to a substrate to form, forexample, a multi-layer laminate sheet flooring product.

Mixing of Thermoplastic Resin and Plasticizer for Solid Compound

Mixing in a batch process typically occurs in a Banbury-type internalmixer operating at a temperature high enough to fuse, or flux, thecombination of PVC and plasticizer. The mixing speeds are typicallyabove 1000 rpm in order to mechanically heat the mixture above thefusion, or flux, point. The output from the mixer is a solid compound inchips or pellets for later calendering into a single layer have athickness useful for making, for example, tile flooring.

Controllable Crosslinking in Compounds

Because the mixture of aromatic dianhydride and epoxy-functionalbioplasticizer is relatively dormant, it is possible to determine themost appropriate time for crosslinking of the bioplasticizer. That eventcan occur before or during melt compounding with a plastisol or a solidplastic resin.

For example, the bioplasticizer can be fully crosslinked during themaking of the crosslinkable bioplasticizer by the steps of mixing,heating, and preferably adding the catalyst to form a crosslinkedbioplasticizer, to be placed in inventory for later use.

Alternatively, the bioplasticizer can remain crosslinkable withoutheating (“green” in rubber chemistry terminology) and then crosslinkedin a later event, such as melt mixing with the thermoplastic resin,whether liquidic or solid and whether in batch or continuous meltprocessing.

Thus, the absence of heat and the optional catalyst remaining separatefrom the mixture of aromatic dianhydride and epoxy-functionalbioplasticizer become a timing determination by a person having ordinaryskill in the art when the crosslinkable bioplasticizer should becrosslinked. The in situ crosslinking during melt compounding of thebioplasticizer and the thermoplastic resin can take advantage of theequipment and techniques used for reactive extrusion, such as thedelivery of the optional catalyst at a port on the extruder downstreamof the throat, in order that the crosslinkable bioplasticizer and thethermoplastic resin can thoroughly mix before crosslinking commences.

The examples below demonstrate that reaction times can vary based on theamount of aromatic dianhydride present in the bioplasticizer. A personhaving ordinary skill in the art of reactive extrusion will appreciatethat the amount of aromatic dianhydride present determines not onlyfinal performance properties but also processing conditions,particularly when reactive extrusion is employed to crosslink thebioplasticizer.

USEFULNESS OF THE INVENTION

Advantages and usefulness of a plasticized thermoplastic compound can beachieved with a crosslinked bioplasticizer which has a much largermolecular weight as a result of the crosslinking. “Blooming” ofplasticizer is less likely to occur from a bioplasticizer which issterically hindered from migration through the thermoplastic resinbecause of its much larger size and conformation arising fromcrosslinking.

Final plastic articles can be made from compounds containing crosslinkedbioplasticizers using extrusion, thermoforming, molding, calendering,and other melt-processing techniques.

Further embodiments are described in the following examples.

EXAMPLES

Table 2 shows the Comparative Examples A-D and Examples 1-5 whichdemonstrated the success of the invention, overcoming failures. EachComparative Example and Example was prepared in a pre-heated cup in anoil bath having a temperature of 150° C.-160° C. Comparative Examples Band D proceeded after the 60 minutes of no reaction of ComparativeExamples A and C, using the same samples, respectively.

Example 1 A B C D 2 3 4 5 Ingredients (Parts) Epoxidized Methyl Soyate100 100 100 100 100 100 100 100 100 Pyromellitic Dianhydride 50 50 25 155 Dodecenyl Succinic 50 50 Anhydride Adipic Acid 50 50 Butyl tintris(2-ethyl Drop* Drop* Drop* Drop* Drop* Drop* hexanoate) (ArkemaFasCat ™ 4102) Results Reaction Duration ~12 None None None None <1 <1<1 ~3 (minutes) after 60 after after 60 after 30** 30*** Form Dark DarkDark Dark Dark Dark Yellow Yellow Yellow Yellow Brown Brown Brown BrownYellow Solid Solid Rubber- Solid Liquid Liquid Liquid Liquid Solid LikeSolid *Approx. 50 mg **After 60 mins. of heating as Comparative ExampleA ***After 60 mins. of heating as Comparative Example C

Table 2 shows in Example 1 that relative dormancy of crosslinking (about12 minutes) for a highly loaded PMDA sample, even when heated, meansthat the duration of “green” crosslinkable condition is slower thaneconomically viable but faster than unheated storage on a shelf betweenproductions. But because the PMDA was readily disbursable into the EMS,probably even dissolving into the EMS, there is no necessity to haveheated that crosslinkable bioplasticizer until minutes before thecrosslinked bioplasticizer is needed.

A comparison of Examples 1 and 2 demonstrated that the presence of thebutyl tin catalyst rapidly accelerates the onset and duration ofcrosslinking reaction, all other factors being equal.

Comparative Examples A and B showed that an aliphatic anhydride was afailure as a crosslinking agent, with or without the butyl tin catalyst.

Comparative Examples C and D showed the same failures for an aliphaticacid, with or without the butyl tin catalyst.

Because Comparative Examples B and D were using the same samples whichhad been heated for 60 minutes previously without catalyst present, thepre-heating and pre-mixing for one hour did not help the conditions forreaction for the additional 30 minutes, even with the catalyst present.Thus, the acid and anhydride were total failures for crosslinkingcapacity with the bioplasticizer.

The progression of Examples 2-5 follows a trend of decreasing content ofaromatic dianhydride, from 50 parts to 5 parts. There is an inflectionpoint in reaction duration between 15 parts and 5 parts, along with aperceptible difference in resulting form. The 5 parts Example 5 may havetaken three times the reaction duration, but it also yielded a morerubber-like solid as compared to the Examples 1-4.

From this series of Examples, one having ordinary skill in the art,without undue experimentation, can tailor the amount of aromaticdianhydride used to achieve faster processing times or more flexible endproducts.

The invention is not limited to these embodiments. The claims follow.

What is claimed is:
 1. A crosslinkable bioplasticizer mixture,comprising: (a) an epoxy-functional bio-derived plasticizer and (b) aneffective amount of an aromatic dianhydride, wherein when heated, themixture crosslinks to form a crosslinked bioplasticizer.
 2. The mixtureof claim 1, wherein the mixture further comprises a Lewis Acid metalcatalyst, and wherein when heated in the presence of the catalyst, themixture crosslinks to form a crosslinked bioplasticizer more rapidlythan the mixture crosslinks without the presence of the catalyst.
 3. Themixture of claim 1, wherein the epoxy-functional bio-derived plasticizercomprises a fatty acid product derived from a vegetable oil having atleast 80% by weight of unsaturated fatty acids, wherein said unsaturatedfatty acids are substantially fully esterified with a monool or apolyol, and said esterified unsaturated fatty acids have beensubstantially fully epoxidized.
 4. The mixture of claim 1, wherein theepoxy-functional bio-derived plasticizer comprises one or moremonoesters or one or more multiesters of epoxidized vegetable oils orcombinations thereof.
 5. The mixture of claim 1, wherein theepoxy-functional bio-derived plasticizer is an epoxy-functional fattyacid ester obtained from a vegetable oil selected from the groupconsisting of soybean oil, canola oil, corn oil, linseed oil, rapeseedoil, safflower oil, sunflower oil, tall oil, tung oil, and mixtures andderivatives thereof.
 6. The mixture of claim 1, wherein theepoxy-functional bio-derived plasticizer is an epoxidized fatty acidester of an unsaturated fatty acid selected from the group consisting ofoleic, linolenic, and linoleic acids, and combinations thereof.
 7. Themixture of claim 1, wherein the epoxy-functional bio-derived plasticizeris an epoxidized soyate selected from the group consisting of epoxidizedpentaerythritol tetrasoyate; epoxidized propylene glycol disoyate;epoxidized ethylene glycol disoyate; epoxidized methyl soyate;epoxidized sucrose octasoyate; the epoxidized product of soybean oilinteresterified with linseed oil; and combinations thereof.
 8. Themixture of claim 1, wherein the aromatic anhydride is pyromelliticdianhydride present in an amount from about 1 to about 50 parts per onehundred parts of the epoxy-functional bio-derived plasticizer.
 9. Themixture of claim 1, wherein the aromatic anhydride is pyromelliticdianhydride.
 10. The mixture of claim 1, wherein the catalyst is butyltin tris(2-ethyl hexanoate).
 11. (canceled)
 12. A thermoplastic compoundcomprising (a) a thermoplastic resin and (b) the crosslinkablebioplasticizer of claim
 1. 13. The compound of claim 12, wherein thethermoplastic resin is selected from the group consisting of polyvinylchloride, polylactic acid, and poly(meth)acrylate.
 14. A method ofmaking a crosslinked bioplasticizer of claim 1, comprising the steps of:(a) mixing the aromatic dianhydride and the epoxy-functional bio-derivedplasticizer in a heated environment and optionally, (b) introducing aneffective amount of Lewis Acid metal catalyst, wherein the aromaticdianhydride and the epoxy-functional bioplasticizer react to form thecrosslinked bioplasticizer.
 15. (canceled)
 16. The method of claim 14,wherein the heated environment of step (a) is between 150° C. and 160°C.